The cerebral cortex is the most complex part of the brain and is responsible for high-level cognitive processing including language, sensory perception, motor planning, attention, emotion and even consciousness itself. To accomplish these complex tasks, the cortex employs an enormous number of heterogeneous neurons. It has long been a mystery how such a diverse population of cells can be generated during development. The immune system also generates enormous cellular diversity, which it achieves by altering the genomic blueprint of certain immune cells in small, but important ways. It has long been speculated that cortical neuron diversity is also generated in part by DNA variation, but direct evidence of such a phenomenon is lacking. In addition to potentially adaptive roles of genomic alterations in cortical neurons, DNA may also be damaged over time. Neurons are particularly susceptible to DNA damage due to their high electrical and metabolic activity and since neurons are not replaced over the course of a lifetime, this damage accumulates as organisms age. DNA damage may play important roles in the pathogenesis of age-related cortical neurodegenerative diseases, such as Alzheimer's Disease and Amyotrophic Lateral Sclerosis. In both adaptive and pathological DNA variation, excision of chromosomal DNA can lead to subsequent formation of circular DNA molecules from these excised pieces. These circular DNAs are found in neurons early in cortical development, in immune cells during genomic editing and as byproducts of DNA damage and thus may be useful markers for underlying DNA variation. The central goal of this proposal is to investigate how DNA alteration may play a functional role in the initial development of diverse cortical neuron types and how pathological DNA damage may accumulate in these cells during aging. Two specific questions drive this goal: 1) If chromosomal excision is involved in generation of neuron types during development, then do neurons of a similar type share similar profiles of circular DNA molecules? 2) Do neurons accumulate class-specific populations of circular DNA from DNA damage during the course of aging? To address these questions, circular DNA from isolated neuronal populations will be sequenced from young and old mice. Additionally, novel techniques will be developed to examine the distribution of circular DNA molecules directly in tissue. The combination of Dr. Paola Arlotta's and Dr. Constance Cepko's guidance, Dr. David Gifford's computational mentorship and expertise, surrounding research community and available resources at Harvard will greatly facilitate this work. Answering these questions would provide novel insights into our basic understanding of cortical neuron development and function and may potentially uncover important drivers of aging and neurodegeneration.